Metallic film forming method using hvof thermal spraying gun and thermal spraying apparatus

ABSTRACT

There are provided a thermal spraying method and a thermal spraying apparatus therefore, in which a gas shroud is disposed in an HVOF thermal spraying gun barrel, the velocity of metallic particles is energized and accelerated with respect to the metallic particle to be thermally sprayed from the gun, and inert gas is supplied into the space defined inside of the shroud through a circumferential slit formed in such a manner as to shield the metallic particles from the atmosphere so as to collide with the surface of a base member, thereby forming a thermally sprayed dense film having a small oxygen content without overheating the base plate by an HVOF method.

This application is a continuation of U.S. application Ser. No.10/515,816, which was the National Stage of International ApplicationNo. PCT/JP2003/012983, filed Oct. 9, 2003.

TECHNICAL FIELD

The present invention relates to a thermal spraying technique, in whichcorrosion resistance or abrasion resistance is applied to the surface ofa base plate by forming a corrosion-resistant thermally sprayed metallicfilm by thermal spraying, so as to prolong the lifetime of a structureor various kinds of industrial equipment and, more particularly, to ametallic film forming method using a high velocity oxy-fuel (hereinafterabbreviated as “HVOF”) thermal spraying gun and a thermal sprayingapparatus for the method.

BACKGROUND ART

It is necessary to prevent (any) corrosion by subjecting a material poorin corrosion resistance to some surface treatment when such a materialis used in seawater or in a coastal environment even if the materialsuch as steel has excellent characteristics as a structural material.Means for preventing (any) corrosion include many methods for painting,plating and the like. However, the painting or plating has raisedproblems yet from the viewpoints of durability and lifetime. On theother hand, an attempt has been made to spray corrosion-resistant powderonto the surface of a base plate by thermal spraying at high temperature(for example, flame spraying, plasma spraying, arc spraying and thelike), so as to fabricate corrosion resistant film. However, a resultantfilm has not been satisfactory in regard to density. Therefore, even ifa technique for forming a corrosion-resistant film has been adopted andcarried out, there has arisen such an ironic result under the currentcircumstances that the film must be subjected to post-treatment bydifferent means, for example, impregnation of a resin in the film orpartial fusion by overheating after the thermal spraying, that is, afterthe formation of the film.

Furthermore, there has been put into practical use acorrosion-prevention method of a sacrificial anode type in which amaterial electrochemically less noble than iron such as zinc or aluminumis coated and selectively eluded, to thus protect a steel base member.In this case, although pores in the film cannot raise any problem, it isconstrued that a corrosion-resistant lifetime can become long if a resinpenetrates the film. However, this has raised a problem of an increasein dissolution rate depending upon the mechanical strength andenvironment of the film, thereby shortening a lifetime of a designedproduct.

In the meantime, a so-called HVOF thermal spraying method has been putinto practical use and has become a focus of attention in recent years,in which material powder is hardly fused but is sprayed to a base memberat a high velocity in a softened state, and then, the powder isinstantly welded by kinetic energy, thereby forming a film. For now,this technique is most used to, for example, an abrasion-resistant(superhard) film made of WC-Co cermet. This is because tungsten carbide(WC) is readily decomposed when it is exposed to, for example, a hotplasma at high temperature; in contrast, WC is much less decomposed at aheat source temperature of about 2,500° C. at the maximum in the case ofthe HVOF, and further, a dense film is formed at a high velocity. Fromthe above-described example, it has been found that the HVOF has thefeature of formation of a dense barrier type film in the atmosphere, andtherefore, has the possibility of formation of a dense film made of acorrosion-resistant material.

In view of such circumstances as described above, the inventors of thepresent application have studied to form dense films made of variouskinds of corrosion-resistant alloys by the HVOF thermal spraying method.As a result, it has been found that even if an Ni-based alloy such asHastelloy is thermal sprayed under a standard condition by acommercially available HVOF thermal spraying apparatus, a considerablydense film excellent in corrosion resistance can be formed. Thisinvention has been patentable already (see Literature 1). Literature 1:Jpn. Pat. No. 3,069,696, “Corrosion-Resistant Thermally Sprayed Film andMethod for Fabricating the Same”

However, a film having satisfactory density could not be made ofstainless steel under spraying conditions in which the commerciallyavailable HVOF thermal spraying apparatus can operate. When thermalpower of combustion flames is increased to enhance the film density, abase member is undesirably overheated, thereby raising a problem ofmarked oxidation of a film. Thus, the present invention has beenaccomplished to solve the above-described problems in a contradictoryrelationship experienced in the prior art according to existing andsimple means. In other words, an object of the present invention is toprovide a thermally sprayed dense metallic film of low oxidation byusing HVOF thermal spraying means without overheating.

BRIEF SUMMARY OF THE INVENTION

As a result of an earnest study conducted by the inventors of thepresent invention, it has been found that a cylindrical attachment(hereinafter referred to as “a gas shroud” or simply referred to as “ashroud”) is attached to a commercially available HVOF thermal sprayinggun, and thereby oxidation of thermally sprayed particles can besuppressed by supplying a great quantity of inert gas into thecylindrical attachment. Further, when the particles are thermallysprayed to the surface of a base plate at an increased particlevelocity, a thermally sprayed dense film with remarkably low oxidationcan be formed without increasing the temperature of combustion flamesvery much. These findings have reached the present invention.

That is to say, according to the present invention, a gas shieldingmeans, which has been already used in the field of plasma spraying, isbasically used in addition to an HVOF thermal spraying gun, and then,the gas shielding means and the HVOF thermal spraying gun are connectedto each other, thereby producing a technical effect which could not beachieved in the prior art. That is, producing an effect of formation ofa thermally sprayed dense metallic film having a small oxygen contentwithout overheating a base plate, with an attendant advantage of aremarkably profound significance. In other words, an object of thepresent invention is to provide an HVOF thermal spraying method equippedwith excellent features based on a series of findings and a thermalspraying apparatus therefor.

Specifically, a first solving means according to the present inventionprovides a metallic film forming method by using an HVOF thermalspraying gun, in which a gas shroud having a cylindrical portion formedinto a shape in conformity with that of a barrel cylindrical portion ofthe thermal spraying gun is disposed in the barrel cylindrical portion.Inert gas is supplied into a space defined inside of the shroud in sucha manner as to suppress oxidation and energize a particle velocity, andmetallic particles are accelerated to collide with a base plate withoutoverheating the base plate, so as to form a thermally sprayed dense filmhaving a low oxygen content at a relatively low temperature. Inaddition, means for supplying the inert gas into the space definedinside of the gas shroud is constituted of a slit formed in acircumferential manner, and can energize the velocity with respect tothe metallic particles to be thermally sprayed by the thermal sprayinggun and prevent mixture of the atmosphere.

Here, the gas shroud to be used has been already used in thermalspraying at high temperature, for example, in plasma spraying. However,the gas shroud has been used merely for controlling the atmosphere andpreventing oxidation of thermally sprayed metal (see Jpn. Pat. Appln.KOKAI Publication No. 224,662/1996), unlike the present invention inwhich the gas shroud is used as means for increasing a particlevelocity. There is no literature which suggests simultaneous achievementof density of a metallic film and a low oxidation by increased velocityachieved by a gas shroud.

A technical report entitled “A Gas Shroud Nozzle for HVOF SprayDeposition” by Pershin V. and three others (on pp. 1305 to 1308 inProceedings of the 15^(th) International Thermal Spraying Conferenceheld at Nice, France from Mar. 25 to 29, 1998) discloses a test resultof the comparative investigation of particle velocities in the casewhere gas is made to flow in a cylindrical gas shroud and where no gasis made to flow therein after the gas shroud of a water coolingstructure having an inside port for introducing nitrogen is disposed inan HVOF thermal spraying gun in such a manner as to surround combustionflames, and in the case where no gas shroud is disposed. As a result, ithas been reported that the velocity of the thermally sprayed metallicparticle is markedly decreased in the thermal spraying gun having thegas shroud disposed therein in comparison with the thermal spraying gunwithout any gas shroud. That is to say, the above-described technicalreport discloses nowhere the suggestion of the achievement of anincrease in particle velocity by disposing the gas shroud in the thermalspraying gun, but merely discloses the utterly contrary result.Moreover, U.S. Pat. Nos. 4,869,936, 5,019,429 and 5,151,308 by Moskowitzand Donald disclose a technique in which a gas shroud is disposed in anHVOF thermal spraying apparatus using hydrogen and oxygen as a heatsource, so as to form a film excellent in corrosion resistance. Thisshroud is adapted to shield thermally sprayed particles from theatmosphere by the use of a swirl flow formed by injecting inert gastoward the inner surface of the shroud from numerous nozzles formedinside thereof, but is not intended (i.e., does not produce any effect)to accelerate the particles.

Furthermore, in order to supply the inert gas into the space definedinside of the gas shroud, a second solving means according to thepresent invention provides a metallic film forming method characterizedin that an inclination with respect to the spraying direction of themetallic particles to be thermally sprayed is provided at the slitformed in the circumferential manner, and thus, the inert gas issupplied into the space defined inside of the gas shroud along theinclination. A third solving means provides a metallic film formingmethod characterized in that the inclination is within an angle of 70°with respect to a line perpendicular to the center axis of the shroudcylindrical portion.

Moreover, a fourth solving means provides a metallic film forming methodcharacterized in that the inert gas supplying means constituted of theslits formed in the circumferential manner are arranged at a pluralityof portions in a lengthwise direction of the gas shroud. A fifth solvingmeans provides a metallic film forming method characterized in that theslits are arranged at two or more portions including a thermal sprayinggun barrel outlet and a gas shroud outlet.

Sixth to tenth solving means relate to a thermal spraying apparatuscorresponding to the above-described solving means according to themetallic film forming means, respectively. Specifically, a sixth solvingmeans according to the present invention provides a thermal sprayingapparatus including an HVOF thermal spraying gun and means for supplyinginert gas into a space defined inside of a cylindrical gas shroud insuch a manner as to suppress oxidation and energize a particle velocityof metallic particles thermally sprayed from the thermal spraying gun,in which the gas shroud having a shape in conformity with that of thebarrel cylindrical portion of the HVOF thermal spraying gun isdetachably attached to the barrel cylindrical portion. Additionally, themeans for supplying the inert gas into the space defined inside of thegas shroud is constituted of a slit formed in a circumferential manner,and can energize the velocity of the metallic particles thermal sprayedby the thermal spraying gun and prevent mixture of the atmosphere.

Additionally, a seventh solving means provides a thermal sprayingapparatus characterized in that an inclination with respect to thespraying direction of the metallic particles thermally sprayed isprovided at the slit formed in the circumferential manner, and thus, theinert gas is supplied into the space defined inside of the gas shroudalong the inclination. An eighth solving means provides a thermalspraying apparatus characterized in that the inclination is within anangle of 70° with respect to a line perpendicular to the center axis ofthe shroud cylindrical portion.

In addition, a ninth solving means provides a thermal spraying apparatuscharacterized in that the slits are arranged at a plurality of portionsin a lengthwise direction of the gas shroud, and a tenth solving meansprovides a thermal spraying apparatus characterized in that the slitsare arranged at two or more portions including a thermal spraying gunbarrel outlet and a gas shroud outlet.

As described above, according to the present invention, the cylindricalgas shroud is attached to the barrel of the HVOF thermal sprayingapparatus, and the thermally sprayed metallic particles are controlledin such a manner as to form the thermally sprayed dense film of a smalloxygen content with the inert gas supplied into the gas shroud withoutoverheating the base plate. This unique configuration can produce thespecial function and effect that the thermally sprayed dense metallicfilm of a small oxygen content can be formed. With this uniqueconfiguration, there has never known yet that the particle velocity ofthe thermally sprayed metallic particle is accelerated, and further, thethermally sprayed dense metallic film of a small oxygen content isformed without overheating the base plate. In addition, the uniqueconfiguration and the special function and effect according to thepresent invention cannot be expected from the test results disclosed inthe prior art literature.

With the above-described configuration, the present invention hassucceeded in achieving the thermally sprayed dense metallic film of asmall oxygen content is formed with reproducibility. Therefore, thepresent invention is a very basic and important invention which widelyinfluences on various kinds of industrial fields in addition to itstechnical significance, thereby socially and economically producing aprominent effect with a remarkably high value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the principle of a high velocityoxy-fuel (HVOF) thermal spraying apparatus;

FIG. 2 is a diagram illustrating the principle of a HVOF thermal sprayapparatus having a gas shroud attached thereto;

FIG. 3 is a diagram illustrating the configuration of the gas shroud ina preferred embodiment;

FIG. 4 is a graph illustrating the relationship between the porosity andoxygen content in thermally sprayed stainless films formed under variousconditions;

FIG. 5 is a graph illustrating the relationship between average particlevelocity of thermally sprayed particles and the porosity of the film;and

FIG. 6 is a graph illustrating the relationship between an iron (i.e.,substrate metal for the Hastelloy alloy thermally sprayed film) ionelusion curve and thermal spraying conditions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A description will be given below of a high velocity oxy-fuel (HVOF)thermal spraying gun and a cylindrical shroud for use in a preferredembodiment according to the present invention. First of all, FIG. 1 is aschematic diagram illustrating the principle of high velocity oxy-fuel(HVOF) thermal spraying. The thermal spraying gun comprises a combustionchamber, a nozzle and a barrel. Fuel and oxygen are mixed and ignitedinside of the combustion chamber, and then, the generated combustionflame is throttled once at a throat, before it passes the divergentnozzle and the straight barrel, to be thus discharged outside. Gas suchas hydrogen, acetylene or propane or liquefied fuel such as kerosene isused as fuel. Material powder is sprayed into the combustion flame at adivergent nozzle outlet with transporting gas by using a negativepressure at the divergent section, and then, is heated and acceleratedinside of the barrel, to be thus discharged to the atmosphere. Thematerial powder normally flies from about 20 cm to about 40 cm in theatmosphere, and then, is deposited on a base plate, thereby forming afilm. Here, mechanical supplying means may be used in place of the meansfor supplying the material powder under the negative pressure.

FIG. 2 is a schematic diagram illustrating the principle of the presentinvention in a state in which a shroud is attached to the HVOF thermalspraying gun illustrated in FIG. 1. FIG. 3 is a schematic diagramillustrating the gas shroud attachment, which consists of a water-cooleddual pipe structure.

Referring to FIG. 2 illustrating the principle of the present invention,in a metallic film forming method by using an HVOF thermal spraying gunaccording to the present invention, a metallic film is formed by theHVOF thermal spraying gun, in which a gas shroud having a cylindricalportion formed into a shape in conformity with that of a barrelcylindrical portion of the thermal spraying gun. Inert gas is suppliedinto a space defined inside of the shroud in such a manner as tosuppress oxidation and energize the particle velocity, and metallicparticles are accelerated to collide with a base plate withoutoverheating the base plate, so as to form a thermally sprayed dense filmhaving a low oxygen content at a relatively low temperature. Means forsupplying the inert gas into the space defined inside of the gas shroudis constituted of a slit formed in a circumferential manner, and canenergize the velocity of the metallic particles to be thermally sprayedand prevent any mixture of the atmospheric air.

Here, it is very important that the means for supplying the inert gasare the slits formed in the circumferential manner. The inert gas to besupplied from these circumferential slits forms a certain kind ofdual-layered acceleration flow in such a manner as to cover thesurroundings of the metallic particles thermally sprayed, energizes thevelocity of the metallic particles, and functions to effectivelysuppress oxidation caused by the mixture of the atmosphere.

Although the slit in this case is preferably formed over the entirecircumference, slits may be intermittently arranged in a circumferentialmanner, or a plurality of slits may be arranged. In order to form theabove-described dual-layered acceleration flow, it is desirable that theinterval between the slits and the length (i.e., the size) of the slitsshould be substantially the same in the arrangement of the slits.

The function and effect of the slits arranged in the circumferentialmanner become prominent owing to an inclined surface formed at the slitin a direction in which the metallic particles travel, as illustrated inFIG. 2.

In the case where the inclined surface is formed at the slit forsupplying the inert gas into the space defined inside of the shroud, itis preferable that the inclination angle should be set within 70° withrespect to a line perpendicular to the center axis of the shroudcylindrical portion.

A plurality of such slits may be arranged in a lengthwise direction ofthe gas shroud. The number of slit arrangement portions or arrangementpositions may be determined in addition to the length of the gas shroudin consideration of the injection rate of the thermally sprayed metallicparticles, the flow rate and quantity of the inert gas and the thicknessand characteristics of the metallic film.

In arranging the slits at the plurality portions as described above,although the above-described inclined surfaces may be adopted at all ofthe slits or not, the formation of at least one inclination surface iseffective. In this case, it is more preferable that the inclinedsurfaces should be formed at two or more portions, that is, at theoutlet of a thermal spraying gun barrel and the outlet of a gas shroudin consideration of the formation of the above-described inclinedsurface at the slit formed at the outlet of thermal spraying gun barrel.

FIG. 3 illustrates the above-described gas shroud in a preferredembodiment. Inert gas (1) and inert gas (2) are supplied into the spacedefined inside of the shroud from two portions. The inert gas (1) isused for mainly accelerating the thermally sprayed particle. A gassupplying port is constituted of a slit formed over the entirecircumference. This slit is arranged in the vicinity of the outlet ofthe thermal spraying gun barrel, and inclined surface at an appropriateangle in a combustion flame injection direction in such a manner as notto interfere the flow of the combustion flame. The other inert gas (2)is used for suppressing the mixture of oxygen from the atmosphere, andis supplied from the slit formed in the vicinity of the outlet of thegas shroud. Incidentally, in the embodiment illustrated in FIG. 3, noinclination is given to the slit for supplying the inert gas (2).

The inert gas for use is exemplified by noble gas such as argon ornitrogen. It is effective that the inner diameter of the shroud isgradually enlarged from the thermal spraying barrel outlet toward theshroud outlet, as in the embodiment illustrated in FIG. 3. In otherwords, the shroud is divergently tapered in the direction of the shroudoutlet. A first reason of the effectiveness of the configuration inwhich the inner diameter of the shroud is gradually enlarged toward theshroud outlet resides in that since a combustion jet is graduallyenlarged toward the atmosphere at the outlet, the turbulence of the flowis small, thereby making it difficult to decrease the velocity. Inaddition, a second reason resides in that the gradually enlarged innerdiameter can prevent occurrence of inconvenience that the thermallysprayed powder adheres to the inner wall of the shroud, which mightcause clogging in a barrel having the same diameter.

Although the configurations of the thermal spraying gun and the shroudfor use according to the present invention have been schematicallydescribed above, they need not be limited to the above-describedconfigurations. As long as a required target is not missed, it isunderstood that variations and additions should be allowed.

Furthermore, the metal or base plate for thermal spraying may beselected from various kinds according to the present invention, andparticularly, it is understood that the base plate may be selected fromvarious kinds such as a flat plate, a curved plate, a bulk member and anodd-form molded product.

Hereinafter, the present invention will be described in more detail byway of preferred embodiments. Of course, the present invention is neverlimited to the preferred embodiments, described below.

First Preferred Embodiment

In the present preferred embodiment, stainless steel (SUS316L) powderwas thermally sprayed by using a high velocity oxy-fuel thermal sprayingapparatus in which a combustion flame of kerosene and oxygen are used asa heat source. The lengths of the barrel were two kinds, that is, 10 cmand 20 cm; nitrogen was used as the inert gas; and thus, the porosityand the oxygen content in the resultant film in each of the barrels weremeasured by varying the combustion condition (i.e., the mixture ratio ofthe fuel to oxygen) and the flow rate of gaseous nitrogen inside of thegas shroud.

Here, the gas shroud had the configuration illustrated in FIG. 3. Theinner diameter on the side of the outlet of the thermal spraying gunbarrel was set to 20 mm; the inner diameter on the side of the outlet ofthe shroud was set to 30 mm; and the length of the shroud was set to 200mm. The inclination surface at an angle of 45° with respect to a lineperpendicular to the center axis of the shroud was formed at the entirecircumferential slit for supplying the inert gas (1) in the vicinity ofthe outlet of the thermal spraying gas barrel. In contrast, noinclination was adopted at the entire circumferential slit for supplyingthe inert gas (2), in which the inert gas was supplied in a directionperpendicular to the center axis of the shroud. A distance from thenozzle outlet to the base plate was set to about 50 cm. Consequently,the distance from the outlet of the shroud to the base plate can becalculated by subtracting the length of the barrel and the length of theshroud from 50 cm. Specifically, in the case where the length of thebarrel was 10 cm, 50−(10+20)=20 cm; or in the case where the length ofthe barrel was 20 cm, 50−(20+20)=10 cm.

The flow rate of the gaseous nitrogen as the inert gas (2) on the sideof the outlet of the gas shroud was constantly 0.45 m³/min.

The supplied quantities and combustion pressures of the fuel and oxygenin oxidizing flame, neutral age flame and reducing flame and otherthermal spraying conditions in the experiments are shown in Table 1below.

Table 2, below, shows below values obtained in an experiment in whichthe length of the barrel and the flow rate of the inert gas (1) for thegas shroud influence on the average velocity and fusion rate of thethermally sprayed particles. These values are results under thecondition where the mixture ratio of the fuel to oxygen achievescomplete combustion. The particle velocity was measured by a non-contactoptical measuring method; and the fusion rate was measured by separatinga fused portion from a not-fused portion by capturing sprayed particleswith an agar gel placed at the position of the base plate (thismeasurement was published and introduced in detail in Journal of theJapan Institute of Metals, 65 (2001) 317-22).

TABLE 1 oxidizing flame neutral flame reducing flame Ox Ne Re keroseneflow rate 0.33 0.41 0.44 (l/min) oxygen flow rate 860 670 600 (stdl/min) combustion pressure 0.65 0.59 0.57 (MPa) barrel length 10, 20(cm) distance between 50 nozzle and base plate (cm) powder supplying 60quantity (g/min) flow rate of shroud 1.5, 2.5 gas 1 (std m³/min) flowrate of shroud    0.45 gas 2 (std m³/min)

TABLE 2 Influence of barrel length and shroud gas flow rate on averagevelocity and fusion rate of thermally sprayed particle barrel flow rateof average particle length shroud gas velocity particle cm std m³/minm/s fusion rate % 10 non 650 42 2.5 672 13 20 non 741 62 1.5 720 43 2.5767 38

The result first reveals that the particle velocity in the case of thebarrel having a length of 20 cm is higher by about 100 m/s than that inthe case of the barrel having a length of 10 cm. When the shroud isdisposed in the barrel, and then, the gaseous nitrogen is made to flowat 2.5 m³/min, the velocity can be further increased by about 20 m/s inboth of the barrels. Incidentally, a flow rate of 1.5 m³/min isinsufficient in the case of the barrel having a length of 20 cm, andtherefore, the velocity is decreased. This results in the findings thatthe flow rate should be desirably 2.0 m³/min or higher in the case ofthe barrel having a length of 20 cm.

Moreover, the particle fusion rate is decreased according to the shroudgas since the introduced gaseous nitrogen has a cooling effect at roomtemperature.

FIG. 4 shows measurement values of the porosity and the oxygen contentin the thermal sprayed stainless steel films obtained under variouskinds of conditions. Arrows in FIG. 4 indicate variations generated bythe use of the gas shroud. In the case where the barrel having a lengthof 10 cm was used (indicated by circles), the oxygen content wasmarkedly decreased under the combustion condition of the neutral flameand the reducing flame; in contrast, the effect was hardly producedunder the combustion condition of the oxidizing flame, and further, theporosity was increased up to 2.5% or more. Since oxygen remains in theuse of the oxidizing flame even if all of the fuel was exhausted, nosuppressing effect on oxidation by the shroud could be expected.

Thus, as for the barrel having a length of 20 cm, a study was made ononly the neutral flame and the reducing flame (indicated by triangles).

Here, numerals “15” and “25” in “Re 15”, “Ne 15” and “No, Re 25” in FIG.4 express shroud gas flow rates 1.5 m³/min and 2.5 m³/min, respectively.

As is clear from FIG. 4, the oxygen content became as great as 3% ormore in the case of no gas shroud. This is because the combustion flamecould approach the base plate when the barrel was long (i.e., 20 cm), sothat the base plate was over heated during the thermal spraying.However, in the case of the use of the shroud, the oxygen contained inthe films could be suppressed down to a remarkably low level by the baseplate cooling effect and the oxidation suppression during the flight ofthe thermally sprayed particles. In addition, when the shroud gas flowrate was 2.5 m³/min with the neutral flame and the reducing flame, theporosity became zero.

FIG. 5 is a graph obtained by re-plotting the porosity data illustratedin FIG. 4 on the lateral axis as the average velocity of the thermallysprayed particle. As is clear from FIG. 5, the particle velocity inexcess of 750 m/s could be obtained by combining the barrel having alength of 20 cm with the gas shroud. In this case, both of the lowoxygen content (i.e., 0.3% or less) and the porosity of zero could beachieved at the same time.

Second Preferred Embodiment

Next, a description will be given below of results that gas shroudthermal spraying is applied to Hastelloy C alloy as one kind ofnickel-based alloys in another preferred embodiment in which anothermaterial is used. It was found that even if the Hastelloy alloy wasthermally sprayed under the standard condition by the commerciallyavailable HVOF thermal spraying apparatus, a considerably dense filmexcellent in corrosion resistance could be formed, and therefore, thisinvention was patentable (Jpn. Pat. No. 3,069,696, “Corrosion-ResistantThermally Sprayed Film and Method for Fabricating the Same”), asdescribed above.

The corrosion resistance in this case was judged from the result thatthe film was soaked in artificial seawater in a laboratory, and then, nocorrosion could not be generated by evaluating the outside appearance, apotential and a corrosion resistance value even after a lapse of threemonths.

However, it was found thereafter that the corrosion resistance wasinsufficient in severe environment such as actual ocean in which waveswash, and therefore, a more severe corrosion resistance evaluation testwas conducted. Specifically, the Hastelloy alloy was thermally sprayedonto carbon steel, and then, an iron ion eluded when it was soaked in a0.5 M HCl solution was quantified by a very sensitive analyzing methodsuch as an ICP (Inductively Coupled Plasma) emission spectroscopy. Atthis time, since the base plate was sealed with a resin, the iron ionsdetected was mainly iron eluded from the base plate through fine poresin the thermally sprayed film (even if it could not be detected by theMercury Porosimeter). This is a more severe evaluation of the density ofthe film. FIG. 6 illustrates the measurement result of variations ofiron ion elusion quantity as the time elapses. FIG. 6 is a graph showingthe measurement results under a standard condition and an HV conditionin addition to the Hastelloy plate material per se. Here, the standardcondition and the HV condition are shown below in Table 3.

TABLE 3 Standard condition HV condition Kerosene flow rate 0.38 0.47(l/min) Oxygen flow rate 862 1080 (std l/min) Combustion pressure 0.680.86 (MPa) Barrel length 10 (cm) Distance between nozzle and 50 baseplate (cm) Thermal spraying distance 38 (cm)

As is clear from FIG. 6, an increase in iron ion elusion quantity wasobserved after about 30 hours in a film obtained by thermal spraying theHastelloy C alloy under the standard condition. Since the temperaturewas constant and no flow occurred in the artificial seawater in thelaboratory, it was construed that a defect inducing the elusion likethat was porously sealed with a generated corrosion product, andtherefore, the corrosion could not proceed. However, as is clear fromthe result illustrated in FIG. 6, it was found that since the iron ioncontinued to be eluded in the actual ocean or severe acidic environment,the corrosion resistance was insufficient. In contrast, a film obtainedunder the HV condition was formed by increasing the quantities of thefuel and oxygen to be supplied to the HVOF thermal spraying apparatus(by about 25% more than that under the standard condition), so as togenerate the higher-velocity combustion flame at a high pressure in thecombustion chamber, and therefore, the resultant film became dense owingto the increase in particle velocity. However, in the case of this film,although an increase in elusion of the iron ion as the time elapsed wassmall, the level of the elusion of the iron ion was not low, andfurther, the iron ion was eluded immediately after the soaking. This wasbecause the film was oxidized much. In the meantime, the elusionquantity from the film obtained by disposing the shroud (indicated bycircles) was substantially the same as the result of the Hastelloy platemember indicated by a dotted line. Therefore, the iron base plate washardly eluded, and the film per se was stable. Like the explanation madeon the stainless steel, the cause was construed to suppress theoxidation of the film by disposing the gas shroud, so as to form thedense and clean Hastelloy film.

Summing up the test results described in the above first and secondpreferred embodiments, the film containing 0.3% or less of oxygen andhaving a porosity of 0 could be formed by using the neutral or reducingcombustion flame and the barrel having a length of 20 cm and adding thegas shroud downstream to allow nitrogen to flow at 2.5 m³/min in thethermal spraying of the stainless steel SUS316L.

Furthermore, it was confirmed that the similar result could be producedeven when the thermal spraying distance was varied from 50 mm to 160 mm(i.e., the distance from the outlet of the shroud tip to the baseplate), or when the angle of the inclination surface was varied from 0°to 70°. Moreover, it was confirmed that the favorable effect could beproduced in the same manner also in the case where the thermally sprayedmetallic particles were sprayed from the outlet of the shroud tipslantwise at an angle of up to 45° to the perpendicular to the baseplate.

Additionally, in the case of HVOF thermal spraying with the Hastelloy Calloy, although the film having a porosity of 0 could be alreadyobtained under the normal thermal spraying condition as evaluated by theconventional Mercury Porosimeter, the corrosion resistance wasinsufficient in the severe corrosion environment (for example, in theactual ocean or in the 0.5 M HCl solution). However, no elusion of theiron ion into an acidic solution was observed by additionally disposingthe gas shroud and thermal spraying the Hastelloy C alloy, and further,the film having the high corrosion resistance was formed.

The principal factor results from the effects that the higher velocityof the thermally sprayed particles, the maintenance of the inertatmosphere, and the suppression of the overheating of the base plate canbe achieved at the same time according to the present invention. Thepresent invention is applicable to other materials, and further, theprinciple based on the useful constituent requirement influences onother thermal spraying methods, or is applicable to other thermalspraying methods as it is.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, the gas shroudis disposed in the HVOF thermal spraying gun and a great quantity ofinert gas is supplied into the space defined inside of the shroud insuch a manner as to suppress the oxidation and energize the particlevelocity, and the metallic particles are made to collide with the baseplate without overheating the base plate, so as to form the thermallysprayed dense film having the low oxygen content at the relatively lowtemperature, so that the function and effect, which could not bepredicted from the test report disclosed in various kinds of literaturereported up to now, can be produced: that is, the velocity of thethermally sprayed metallic particles can be increased, and the thermallysprayed dense metallic film having the low oxygen concentration can beformed without over heating the base plate. Therefore, the presentinvention is an epoch-making technique which can break a barrierexperienced by the prior art. In addition to the remarkable enhancementof the corrosion resistance of the steel structural member and variouskinds of equipment, the present invention is expected to be widelyutilized in various fields including coating on the welds or ends of thevarious types of clad members, as well as repairing of damaged portion.Furthermore, the present invention is expected to provide an effectivemean to prolong the lifetime of various steel structure of greatindustrial and economic importance.

1. A metallic film forming method using a high velocity oxy-fuel (HVOF)thermal spray gun having a cylindrical barrel portion extending in alongitudinal direction towards a base plate, the cylindrical barrelportion having an outlet, the method comprising: connecting a gas shroudto the outlet of the cylindrical barrel portion in a coaxial manner, thegas shroud having a substantially cylindrical portion extending in thelongitudinal direction with an outlet at one end and an inlet at anotherend and having a length longer than a diameter of the inlet, the gasshroud being connected to the outlet of the cylindrical barrel portionsuch that the inlet of the gas shroud and the outlet of the cylindricalbarrel portion are in conformity with each other, an inner diameter ofthe substantially cylindrical portion of the gas shroud beingcontinuously enlarged from the outlet of the cylindrical barrel portionto the outlet of the gas shroud; spraying metallic particles from thethermal spray gun by a neutral flame or a reducing flame such that themetallic particles collide with the base plate; and supplying inert gasto a space within the gas shroud through a slit formed at the outlet ofthe cylindrical barrel portion and in a circumferential manner in thegas shroud, the supplied inert gas suppressing oxidation of the metallicparticles by preventing any mixture of the metallic particles with theatmosphere, and the supplied inert gas increasing a velocity of themetallic particles so that the metallic particles collide with the baseplate without overheating the base plate, to thereby form a thermallysprayed dense film having a low oxygen content.
 2. The metallic filmforming method of claim 1, wherein the slit is arranged at aninclination with respect to a spraying direction of the metallicparticles such that the inert gas is supplied into the space within thegas shroud along the inclination.
 3. The metallic film forming method ofclaim 2, wherein the inclination is within 70° with respect to a lineperpendicular to a center axis of the cylindrical portion of the gasshroud.
 4. The metallic film forming method of claim 3, wherein the slitis arranged at a plurality of portions in the longitudinal direction ofthe gas shroud.
 5. The metallic film forming method of claim 2, whereinthe slit is arranged at a plurality of portions in the longitudinaldirection of the gas shroud.
 6. The metallic film forming method ofclaim 1, wherein the slit is arranged at a plurality of portions in thelongitudinal direction of the gas shroud.
 7. The metallic film formingmethod of claim 6, wherein the slit is arranged at least at the inlet ofthe gas shroud and at the outlet of the gas shroud.
 8. A thermalspraying apparatus comprising: a high velocity oxy-fuel (HVOF) thermalspray gun for thermally spraying metallic particles to a base plate,said thermal spray gun having a cylindrical barrel portion extending ina longitudinal direction, said cylindrical barrel portion having anoutlet, and said thermal spray gun having a heat source comprising aneutral flame or a reducing flame; a gas shroud having a substantiallycylindrical portion extending in the longitudinal direction with aninlet at one end and an outlet at another end and having a length longerthan a diameter of the inlet, said gas shroud being detachably connectedin a coaxial manner to said outlet of said cylindrical barrel portionsuch that said inlet of said gas shroud and said outlet of saidcylindrical barrel portion are in conformity with each other, wherein aninner diameter of said substantially cylindrical portion of said gasshroud is continuously enlarged from said outlet of said cylindricalbarrel portion to said outlet of said gas shroud; and said gas shroudhaving a slit for supplying inert gas into a space within said gasshroud, said slit being formed in said gas shroud at the outlet of thecylindrical barrel portion and in a circumferential manner, wherein saidthermal spray gun, said gas shroud and said slit are structured andarranged such that the inert gas supplied through said slit increases avelocity of the metallic particles being sprayed from said thermal spraygun, and suppresses oxidation of the metallic particles by preventingany mixture of the metallic particles with the atmosphere.
 9. Thethermal spraying apparatus of claim 8, wherein said slit is arranged atan inclination with respect to a spraying direction of the metallicparticles such that the inert gas is supplied into said space withinsaid gas shroud along said inclination.
 10. The thermal sprayingapparatus of claim 9, wherein said inclination is within 70° withrespect to a line perpendicular to a center axis of said substantiallycylindrical portion of said gas shroud.
 11. The thermal sprayingapparatus of claim 10, wherein said slit is arranged at a plurality ofportions in the longitudinal direction of said gas shroud.
 12. Thethermal spraying apparatus of claim 9, wherein said slit is arranged ata plurality of portions in the longitudinal direction of said gasshroud.
 13. The thermal spraying apparatus of claim 8, wherein said slitis arranged at a plurality of portions in the longitudinal direction ofsaid gas shroud.
 14. The thermal spraying apparatus of claim 13, whereinsaid slit is arranged at least at said inlet of said gas shroud and atsaid outlet of said gas shroud.
 15. The metallic film forming method ofclaim 1, wherein a porosity of the thermally sprayed dense film is zero.16. The metallic film forming method of claim 15, wherein the metallicparticles include stainless steel powder.
 17. The thermal sprayingapparatus of claim 8, wherein the metallic particles include stainlesssteel powder.
 18. The method of claim 1, wherein the thermal spray gunfurther includes a combustion chamber and a divergent nozzle, fuel andoxygen mixing and igniting inside of the combustion chamber to form acombustion flame that passes through the divergent nozzle and passesfrom the divergent nozzle into the cylindrical barrel portion.
 19. Themethod of claim 18, wherein said spraying metallic particles comprisesspraying material powder into the combustion flame at an outlet of thedivergent nozzle.
 20. The apparatus of claim 8, wherein said thermalspray gun further comprises a combustion chamber and a divergent nozzlesuch that fuel and oxygen can be mixed and ignited inside of thecombustion chamber to form a combustion flame which then passes throughsaid divergent nozzle to said cylindrical barrel portion and whereinsaid divergent nozzle has an outlet at which material powder can besprayed into the combustion flame.